The United Nations nuclear watchdog said on Monday it had received an invitation from North Korea to visit, three years after its inspectors were expelled from the reclusive communist state for a second time.

The International Atomic Energy Agency, the Vienna-based UN body tasked with preventing the spread of nuclear weapons in the world, said it received the invitation on March 16 but declined to give details regarding its content.

"We will discuss with the DPRK (North Korea) and other parties concerned for the details of the visit," IAEA spokeswoman Gill Tudor said in an e-mailed statement. "Nothing has been decided yet."

In Washington, the State Department said that in principle it supported all efforts by the IAEA to gain access to North Korea to monitor Pyongyang's implementation of all aspects of the Feburary 29 nuclear agreement.

The move follows a nuclear moratorium deal with the United States last month in which Washington agreed to supply the North with food in exchange for a suspension of nuclear tests, missile launches and uranium enrichment and to allow IAEA inspectors back into the country.

But Pyongyang's announcement, on same day the IAEA received its invitation, that it planned to launch a rocket carrying a satellite to mark the centenary of founder Kim Il-sung's birth next month drew swift international condemnation.

The United States said the North's plan to launch a satellite could violate its February nuclear moratorium agreement and scuttle the resumption of US food aid.

'Violation of obligations'

"Obviously there's benefit for any access that the IAEA can get," State Department spokeswoman Victoria Nuland said on Monday.

"But it doesn't change the fact that we would consider a satellite launch a violation not only of their UN obligations but of the commitments they made to us."

South Korea condemned rival North Korea's planned rocket launch as a "grave provocation," saying it was a disguised attempt to develop a long-range ballistic missile capable of delivering nuclear weapons.

Japan's Kyodo news agency on Monday quoted a senior North Korean official as saying North Korea had asked the IAEA to send inspectors to monitor the moratorium of its uranium enrichment programme.

But a Vienna-based Western diplomat told Reuters that North Korea had "offered to meet with the IAEA to discuss the moratorium; they didn't actually offer the IAEA moratorium monitoring at this point."

The secretive North has twice tested a nuclear device, but experts doubt whether it yet has the ability to miniaturise an atomic bomb to place atop a warhead.

Pyongyang is believed to have enough fissile material to make up to a dozen nuclear bombs, and in 2010 unveiled a uranium enrichment facility to go with its plutonium programme which opened a second route to making an atomic weapon.

North Korea expelled the IAEA a decade ago when a 1994 deal between Pyongyang and Washington unravelled.

It threw the organisation out again in April 2009 after rejecting the intrusive inspections provided for under a 2005 aid deal with five regional powers that allowed the UN watchdog to return.

It is unclear how much scope for inspections the IAEA will get. The North has limited their access during two previous periods when it allowed inspectors in.